Mass air flow meters used in automotive vehicles often are of the constant temperature anemometer type. In these meters, a sensing element is electrically heated to a constant temperature differential above ambient air temperature. Heat is convectively removed from the element by the airflow and the curren flowing in the element replaces the heat lost by convection. As the mass air flow varies, the current required to maintain the requisite temperature also varies such that the current is a known function of the mass air flow.
Since the convective heat loss is not the only loss mechanism, inaccuracies can creep into airflow measurements. Heat is also removed from the sensor by radiation to the surroundings and conduction to the sensor support. Radiation loss is a function of area of the heated element and can be minimized by making the sensor very small. The heat loss by conduction, on the other hand, has been a source of inaccuracy in such meters, particularly since it varies in accordance with air flow and also in accordance with the temperature of the supporting structure. In the case of a cylindrical sensor body with terminal fittings on the ends, heat is conducted from the sensor to the terminal fittings and thence to the support which mounts the fittings. At high airflow rates the heat is removed convectively from the terminal fittings so that they do not effectively transfer heat to the supports but at low airflow rates the terminals and support become warm. Thus the sensor element effectively becomes larger at the low rates and the current to airflow function becomes skewed. This restrists the dynamic range for accurate operation to higher airflow rates. To avoid the meter inaccuracies engendered by this thermal characteristic, it is desirable to account for the heat loss by conduction or to prevent heat loss by conduction.
An important characteristic of a heat sensor is its response time. A sudden change of air flow or ambient temperature should be quickly reflected in the heating current. When the sensor has a large mass, it does not change temperature as fast as a small mass sensor. When sensor terminals are heated by conduction they effectively add to the mass and to the response time. Thus, optimizing response time is another reason for preventing heat loss from the heated element to the terminals.
The desirability of controlling thermal characteristics of such sensors has been recognized in U.S. Pat. No. 4,587,842 to Handtmann. There, the heater coil is wound on a massive support and is bounded at either end by an auxiliary heater controlled to the same temperature as the heater coil. The auxiliary heaters maintain the temperature of the support so that there is no heat transfer from the coil to the support, resulting in fast response.
U.S. Pat. No. 4,559,814 to Sato et al describes the heat sink effect of the terminal leads fastened in the ends of a tubular support for a heated sensor and particularly the resulting temperture variation along the length of the support and the coil. A uniform temperature is achieved by winding the resistor onto the support coarsely at the center and densely at the ends. The resistor is wound as close to the end of the support as is practical. This patent, like Handtmann does not teach how to prevent or even to compensate for the loss of heat to the leads or terminals.